How to create a shape in the UV image editor for using as a mask in VSE?
What you want to do is split a screen and open a UV/Image Editor, or just open one up through the bottom, lefthand icon:



Load in an image with the Open button (you can screenshot a frame of your video) so that you know where to put your mask.



Still in the footer of the UV/Image Editor, add switch from View mode to Mask mode.

 --> 

Add a new mask:



Now there are three keys to remember:


Ctrl +  LMB adds a Control Point where you click.
Once you have three or more points, use A to select all/deselect all.
If every point is selected (yellow), use Alt + C to connect the points.